disciplines incline their members to move in one direction rather than another. The principal problem to be addressed, however, is not one linked to the sociology of knowledge. I want to pose the question, "Who migrates?" because I believe that a satisfactory understanding of the role of migration in developing countries must depend upon some knowledge of the forms and degree of migrant selectivity. Migrant selectivity is broadly defined so as to include the factors that distinguish migrants from the general population—not only demographic variables, such as age and sex, and socioeconomic variables, such as education and occupation, but even such elusive psychological characteristics as the propensity to assume risk. Our task is not ended with the establishment of a set of migration differentials for various developing countries. Migrant selectivity for a given group should not be considered as some invariant property; it may change over time in response to changed conditions. The relationship of migrant selectivity to the urbanization process is a case in point. Increasing urbanization in a country affects the rate and degree of selectivity of rural-urban migration. There are many problems of data and method in a longitudinal approach, but migrant selectivity must be put within a dynamic framework, a context lacking in most studies of this phenomenon. Restrictions on Scope and Coverage This investigation is restricted in several ways. Attention is not primarily directed to all forms of internal migration nor even to rural-urban migration but is confined to migration to the large cities of developing countries. More information is available for this class of cities. Small and medium-size cities up to 100,000 have not received nearly the attention given the larger cities. Even among the latter there is generally an emphasis on the first city or the first two cities. In Latin America, where first-city dominance is especially pronounced, the index of entries by city in a recent bibliography on urbanization in Latin America by Vaughan (4) shows that for nearly every country the first city, or in a few cases the first two cities, account for more entries than all other cities combined. There are more important reasons for limiting our attention to the large cities. As a group they are growing very rapidly and represent increasingly large shares of both the urban and the total populations of their countries. Put another way, most developing countries are metropolitanizing (growth in places over 100,000) at a faster rate than they are urbanizing, and at a faster pace than was true for most western nations at comparable stages in their urbanization. Table 1 provides support for this statement. Note that in nearly all regions the urban population is growing at least twice as fast as the rural population and the "city" rate is appreciably greater than the "town" rate.